Such micromechanical sensors are known for detecting accelerations of rotational and translational movements. They are typically fabricated on the basis of conventional etch techniques from a wafer, for instance made of silicon, and generally comprise a substrate forming a supporting structure, which during an intended use of the acceleration sensor is mostly firmly connected to a system whose acceleration is to be detected. The substrate usually represents the reference system for the sensor the acceleration of which is to be determined. By means of a support structure a mass unit is resiliently connected with the substrate and is symmetrically or asymmetrically arranged. In the former case, for example, it may centrally be suspended via springs at four anchors that are outwardly positioned. In the latter case at least a pivot axis is defined around which the mass unit performs a rotation relatively to the substrate due to an acceleration acting on the sensor. The mass unit is then configured such that it has a center of gravity that is offset from the corresponding pivot axis. Furthermore, the sensor has in each case at least a detection unit, by means of which a change in position between the mass unit and the substrate caused by an acceleration acting on the sensor is detectable.
The center of gravity of the mass unit offset from the pivot axis is, for example defined in such a manner that the mass unit is axis-asymmetrically configured with respect to the pivot axis or the mass unit consists of material of different density. The mass unit is connected with the corresponding detection units in a conventional manner by means of a frame structure formed of bar-like components of the wafer.
Disadvantageously the mass unit forming the inertial mass suffers from a relatively strong shape specific thermal expansion behavior due to its asymmetric shape or its differing expansion behavior of possibly existing different materials or cover layers. Furthermore, stresses caused by the manufacturing are present in the wafer material or any other material, which stresses are released via the shape of the sensor. This means that the shape of the mass unit deviates from a shape required for a precise detection of accelerations, which is increased in particular in the presence of temperature variations. In this case the mass unit does not expand in a uniform manner but the occurring expansions strongly differ in different areas. The inner stresses and the shape specific thermal expansion behavior of the mass unit therefore result in particular in a twisting or distortion of the mass unit, wherein it partly significantly bends out of the main extension plane of the sensor.
The non-uniform changes in shape or expansions of the mass unit are disadvantageously transferred to the structure connecting the mass unit with the remaining components of the sensor and are transferred in particular to the detection units for detecting the change in position between the mass unit and the substrate. Therefore, partly pronounced measurement inaccuracies as well as temperature dependent measurement differences are obtained. These effects are in particular troublesome in acceleration sensors serving for the detection of small accelerations. Depending on the correspondingly used materials and the particular configuration of the structure of the sensor also at an acceleration of 0 G strong variations may occur across the entire temperature range, thereby making a tuning of the sensing virtually impossible.
Based on the prior art described above and the resulting disadvantages, it is an object of the present invention to provide a sensor for detecting accelerations, in particular a micromechanical sensor as described above, in which variations of measurement readings in particular due to the release of internal stresses as well as due to the occurrence of temperature dependent non-uniform expansions may mostly be avoided. In preferred embodiments, the sensor would be configured to detect the acceleration along two axes that are mutually orthogonal to each other.